Process and compositions for forming improved phosphate coatings on metallic surfaces



United States Patent 3,146,133 PROCESS AND COMPQSETIQN S FOR FOGIMkROt/ED PHGSPHATE COATINGS ON ME- TALLllC SURFACES Jean Lantoin,Sucy-en-Brie, France, assignor, by mesne assignments, to Hooker ChemicalCorporation, New York, N.Y., a corporation of New York No Drawing. FiledMar. 12, 1962, Ser. No. 179,205 Claims priority, application France Mar.23, 1961 5 Claims. (Cl. 148-615) The present invention relates to theart of forming adherent, integrally bound coatings on metallic surfaceswhich are useful as protection for said surfaces against corrosion, as abase for paint and to protect the surfaces during metal deformationoperations and the like. More particularly, this invention relates to animproved method for forming modified phosphate coatings on the surfacesof commercial metals which are susceptible to atmospheric corrosionparticularly the surfaces of irons and alloys thereof.

At the present time, in the art of formation of phosphate coatings onmetal surfaces there are two main types of solutions which are in use.The first of these solutions employs the dihydrogen phosphate of a heavymetal, particularly zinc and manganese and these aqueous acidicsolutions form a coating in which the metal of the solution forms anintegral part of the coating on the metal surface, that is, the coatingincludes zinc or manganese. Such phosphate coatings are crystalline innature and are fairly heavy coatings having a thickness on the order ofabout grams per square meter. The crystalline nature of such coatingsmay be revealed either by looking at it with the naked eye or under amicroscope at a magnification of up to about 1200 diameters. It is knownthat the size of the crystals has an important influence on theabsorption qualities on the coating and its ability to form a tightadherent base for organic finishes, such as paints, lacquers, or thelike, and it is additionally known that the crystalline size of theparticles within such a coating may be refined by addition to thesolution of oxidizing agents or other additives such as the condensedphosphates or the like, and by the addition of such additives, thecoating weight can be reduced to within the range of about 1.5 to about3 grams per meter and with a concurrent reduction in the size of thecrystals in the coating.

The second type of process makes use of the dihydrogen phosphates of thealkaline earth metals or of ammonium phosphates or organic phosphates.Solutions of the alkaline earth metals or ammonium or organic phosphatesare aqueous acidic solutions which form on a metal surface, for example,iron, a coating which has a much finer particle size than that which wasobtained from zinc and manganese dihydrogen phosphate solutions. Theparticle size of such coatings is reduced to such a degree that they areconsidered amorphous-like coatings because an examination of them undera microscope at a magnification up to about 1800 diameters does notreveal a true crystalline structure. Such coatings are ordinarilythinner and fall into the range of 0.5 to about 0.8 grams per squaremeter.

Each of these two types of processes are advantageous for certain usesand inconvenient for other types of uses. Coatings from the dihydrogenphosphates of zinc and manganese provide good protection againstcorrosion and are particularly good in aiding in the cold forming ofmetal surfaces. However, such heavy zinc or manganese phosphate coatingsdo not provide as good a base for paint as the coatings of the secondtype. The second type of coating solution produces a coating whichprovides improved adherence for paints, lacquers, enamels, or the like,but such coating is far less satisfactory as far as resistance tocorrosion or as an aid in the deformation of the metal coated with thatlight weight amorphous-like coating.

The present invention provides a solution and a method for formingcoatings which combines the advantages of the above two types ofcoatings and avoids their disadvantages. In accordance with thisinvention, it has been found that the addition of aluminum ions to anaqueous acidic solution of zinc or manganese phosphate produces arefinement of the crystals and a reduction in their size which placesthem in the category of the second type of coatings, namely,amorphous-like coatings. The incorporation of aluminum ions in theaqueous dihydrogen zinc and manganese phosphate solutions of thisinvention has been found to cause a substantial reduction in thethickness or weight of the coating which is formed relative to thecoating which would be formed by an otherwise identical solution exceptthat it contains no aluminum, and the coatings of this invention havethe Weight in the range between about 0.8 and 1.6 grams/sq. meter orotherwise stated, about to about 150 milligrams per square foot, and thepreferred coatings have a weight in the range of about 1 gram to 1.3grams/ sq. meter or to milligrams per square foot. Such amorphous-likecoatings have been found to be particularly satisfactory in resistingcorrosion and to be substantially improved as a base for paint, lacquer,or the like, and moreover, to provide good protection to the metalsurface during formation operations such as forming, rolling, stampingand the like.

The basic solution which is improved by incorporation of the aluminumion thereby may be any of the heretofore known aqueous acidic zinc ormanganese phosphate solutions. Such solutions typically contain about0.5 to about 2.5% P0 sufiicient zinc or manganese to form the dihydrogenphosphate and has a total acid in the range of about 10 to about 50points. The total acid expressed in points, refers to the number of ml.of N/10 sodium hydroxide which is required to neutralize a ten ml.sample of the operating solution to a phenolphthalein end point.

The phosphate coating solution should contain an oxidizing agent whichis capable of accelerating the rate of coating formation, or metalattack, so that a uniform coating can be obtained in a reasonably shortperiod of time, for example, about 1-5 minutes. For this purpose,nitrite is the preferred accelerating agent and may be present in anamount from about 0.002% to about 0.015%. Other suitable oxidizingagents include about 0.03 to 0.05% bromate, 0.02%0.l5% sulfite, 0.05.25%sodium meta nitrobenzene sulfonate, 0.05-0.25 nitro methane and 0.050.25nitro guanidine, 0.0*l%4% nitrate, 0.54% chlorate, 0.0 l%-1% sodiumethylene diamine tetraacetate, or mixtures thereof, and particularlymixtures of nitrate and nitrite.

The aluminum ions may be introduced into the solution in the form of anyaluminum salt or compound that is soluble in the aqueous acidic solutionof zinc phosphate and which will not destroy the oxidizing agent ordetrimentally affect the coating-forming ability of the solution.Typical examples of aluminum compounds which are suitable for thispurpose include aluminum acetate, aluminum sulfate, aluminum phosphate,aluminum carbonate, aluminum fluoborate and aluminum hydroxide. It ispreferred to introduce the aluminum into the solution in a form whichadds anions already present in the solution, for example, phosphates,and it is desirable to avoid anything other than minor quantities ofchloride, chromate or arsenate anions.

The aluminum ion should be present in the solution in a proportion ofabout 0.2 gram to about grams per liter and preferably in aconcentration in the range of about 0.04 gram to about 1 gram per liter.The stability and equilibrium of the solution, during operation, is bestassured when the weight ratio of aluminum to zinc is between 1 to 7 and1 to 40 and preferably is between 1 to 20 and 1 to 30.

The solutions of this invention are suitable to form coatings over awide range of temperatures and will form coatings at all temperaturesbetween normal room temperature and the boiling point of the solution,but are preferably operated in the range of about 70 F. to 120 F. Thesolutions may be applied by brushing, dipping, spraying or atomizing orthe part to be coated may be immersed in the solution maintained in theabove given temperature range. After the phosphate coating is formed,the coating is preferably rinsed in a dilute aqueous chromic acidsolution, or one which contains the hexavalent chromium ion, and asuitable concentration for such a solution is one which contains theequivalent of about 2 to about 4 ounces of CrO per 100 gallons of water.

The folowing examples are given to illustrate in somewhat greater detailtypical formulations which are suitable for use in accordance with thisinvention, but it is to be understood that the examples are given forillustrative purposes only and do not represent the definitive limits ofthe invention which have been described hereinabove.

Example 1 An aqueous solution was prepared by adding per liter of water:

Grams/liter Zn(H PO 9.2 Al(H P0 3.6 P1 20 4.8 Zn(NO 18.5 NaNO 13.2

In terms of ions, this composition contains 15.1 grams per liter P0 21.7grams per liter N0 8.6 grams per liter Zn and 0.3 gram per literaluminum. The solution had a total acidity of 30 points and a pH of 2.3.

Cold rolled steel panels were preliminarily cleaned in conventionalfashion to remove grease and dirt and then immersed in the abovesolution for 7 minutes at 80 C. The panels were then removed, rinsedwith cold water and thereafter rinsed in a dilute chromic acid solutioncontaining 3 ounces of CrO per 100 gallons of water, removed, rinsed anddried. An inspection of the coating showed that the coating was uniform,adherent and microscopic examination of the coating revealed that theparticles were much smaller than the particles obtained from solutionsotherwise similar but which do not include the aluminum ion, theparticles being generally characterizable as amorphous-like ormicrocrystalline. The coating weight was determined to be 1.2 grams persquare meter.

Another bath was prepared, identical to the above, except that itcontained no aluminum and panels coated in this solution underconditions identical to those specified above were found to be coatedwith a coating in the range of 67 grams per square meter and theparticles of that coating were found to be typical large grain crystals.Panels from the aluminum containing solution were painted inconventional fashion, and subjected to salt spray corrosion andaccelerated humidity tests, and in all cases were found to be at leastequivalent to similarly painted panels coated with zinc phosphateproduced from solutions containing no aluminum. The painted panels weresubjected to the conventional scratch adhesion test, impact test, andbend test and in all cases the results were satisfactory.

4 Example II An aqueous solution was prepared to contain:

Grams/liter r0. 17.3 No 2.75 Zn 5.5 Al .7 C10 4.7

This solution has a total acidity of 30 points and a pH of 2.2.

Conventionally cleaned cold rolled steel immersed in this bath at 60-65C. for 12 minutes were found to be coated with an adherent coatinghaving a weight of about 1.12 grams per square meter.

Example III A solution was prepared containing:

Grams/liter P0 10.5 N0 4.2 Zn 5.8 A1 .2 Na .34 N0 .25

This soltuion had a pH of 2.8 and a total acidity of 16 points. Thesolution was applied to cold rolled steel panels by pressure spraying atabout 45 C. and an inspection of the coating showed it to have a generalappearance similar to that obtained in Example I and a coating weight of1.0 grams per square meter.

What is claimed is: p

1. A process for coating the surface of a metal which is susceptible toatmospheric corrosion which comprises applying to said surface anaqueous acidic solution consisting essentially of a phosphate selectedfrom the group consisting of zinc and manganese phosphates, the P0content of said solution being in the range of .5 to about 2.5%, and atleast one oxidizing agent selected from the group consisting of 0.002%to 0.015% nitrite, 0.03 to 0.05% bromate, 0.02%0.15% sulfite, 0.050.25%sodium metal nitrobenzene sulfonate, 0.050.25% nitro methane, 0.050.25%nitro guanidine, 0.01%4% nitrate, 0.5- 4% chlorate and 0.01%-1% sodiumethylene diamine tetra-acetate, and the aluminum ion in an amount in therange of about 0.02 to about 5 grams per liter, said solution having atemperature in the range of room temperature to the boiling point ofsaid solution.

2. A process in accordance with claim 1 wherein said phosphate is zincphosphate and the weight ratio of aluminum to zinc is in the range of 1to 7 to 1 to 40.

3. A process in accordance with claim 1 wherein said phosphate is zincphosphate and the weight ratio of aluminum to zinc is in the range of 1to 20 to 1 to 30.

4. A process in accordance with claim 1 wherein said metallic surface isa ferrous surface and the said solution is applied thereto so as to forma coating on said surface having a weight in the range of about to about150 mg./ sq. ft.

5. A process in accordance with claim 1 wherein said metallic surface isa ferrous surface and the said solution is applied thereto so as to forma coating on said surface having a weight in the range of about to aboutmg./ sq. ft.

References Cited in the file of this patent UNITED STATES PATENTS2,316,810 Romig Apr. 20, 1943 2,375,468 Clifford et al May 8, 19452,540,314 Amundsen Feb. 6, 1951 2,554,139 Drysdale May 22, 19512,743,204 Russell Apr. 24, 1956 2,758,949 Ley et al. Aug. 14, 1956UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 146,133 1 August 25 1964 Jean Lantoin It is hereby certified that errorappears in the above numbered patent requiring correction and that thesaid Letters Patent should read as corrected below.

Column 2 line 34, for "thereby" read therein column 3, line 7, for "0,2"read 0.02

Signed and sealed this 30th day of March 1965::

(SEAL) Attest:

ERNEST w. SWIDER EDWARD J. BRENNER Aitesting Officer Commissioner ofPatents

1. A PROCESS FOR COATING THE SURFACE OF A METAL WHICH IS SUSCEPTIBLE TOATMOSPHERIC CORROSION WHICH COMPRISES APPLYING TO SAID SURFACE ANAQUEOUS ACIDIC SOLUTION CONSISTING ESSENTIALLY OF A PHOSPHATE SELECTEDFROM THE GROUP CONSISTING OF ZINC AND MANGANESE PHOSPHATES, THE PO4CONTENT OF SAID SOLUTION BEING IN THE RANGE OF .5 TO ABOUT 2.5%, AND ATLEAST ONE OXIDIZING AGENT SELECTED FROM THE GROUP CONSISTING OF 0.002%TO 0.015% NITRITE, 0.03 TO 0.05% BROMATE, 0.02-0.15% SULFITE, 0.05-0.25%SODIUM METAL NITROBENZENE SULFONATE, 0.05-0.025% NITRO METHANE,0.05-0.25% NITRO GUANIDINE, 0.01%-1%-4% NITRATE, 0.54% CHLORATE AND0.01%-1% SODIUM ETHYLENE DIAMINE TETRA-ACETATE, AND THE ALUMINUM ION INAN AMOUNT IN THE RANGE OF ABOUT 0.02 TO ABOUT 5 GRAMS PER LITER, SAIDSOLUTION HAVING A TEMPERATURE IN THE RANGE OF ROOM TEMPERATURE TO THEBOILING POINT OF SAID SOLUTION.